Structural and dynamical aspects of uranyl ions in supercritical water: A molecular dynamics simulation study

Abstract

Water and aqueous solutions of radioactive ions at supercritical conditions are of immense importance due to their applications in nuclear industry. All atom molecular dynamics simulations are carried out to investigate structure and dynamics of various species present in bulk water as well as aqueous solutions of uranyl ions at supercritical conditions. Although with the increase in density of water (or pressure), coordination/hydration numbers of the ions as well as water increase, local short ranged order around water as well as different ionic species decreases with increasing water densities (or pressure). However, orientational structures of water in the first solvation shell of another water molecule or ion remain almost invariant with increase in water density. Translational diffusivities of different species as obtained from respective mean squared displacements have been found to decrease with increasing water densities. The diffusivity values of water molecules in bulk supercritical water at different densities as extracted from the present simulation study have been found to agree quite well with the corresponding experimental results. As expected, orientational relaxations of different species are found to be slowed down with increasing density of water both in pure water and in solutions. It is in general observed that the translational and orientational dynamics of water and ionic species are faster at a supercritical temperature as compared to the same at corresponding normal state.